Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Nanomaterials, such as metal or semiconductor nanoparticles and nanorods, exhibit similar dimensions to those of biomolecules, such as proteins (enzymes, antigens, antibodies) or DNA. The integration of nanoparticles, which exhibit unique electronic, photonic, and catalytic properties, with biomaterials, which display unique recognition, catalytic, and inhibition properties, yields novel hybrid nanobiomaterials of synergetic properties and functions. This review describes recent advances in the synthesis of biomolecule-nanoparticle/nanorod hybrid systems and the application of such assemblies in the generation of 2D and 3D ordered structures in solutions and on surfaces. Particular emphasis is directed to the use of biomolecule-nanoparticle (metallic or semiconductive) assemblies for bioanalytical applications and for the fabrication of bioelectronic devices.

Integrated Nanoparticle–Biomolecule Hybrid Systems: Synthesis, Properties, and Applications

Annual review of condensed matter physics

Functional π-Gelators and Their Applications

A possible sighting of Majorana states Nearly 80 years ago, the Italian physicist Ettore Majorana proposed the existence of an unusual type of particle that is its own antiparticle, the so-called Majorana fermion. The search for a free Majorana fermion has so far been unsuccessful, but bound Majorana-like collective excitations may exist in certain exotic superconductors. Nadj-Perge et al. created such a topological superconductor by depositing iron atoms onto the surface of superconducting lead, forming atomic chains (see the Perspective by Lee). They then used a scanning tunneling microscope to observe enhanced conductance at the ends of these chains at zero energy, where theory predicts Majorana states should appear. Science, this issue p. 602; see also p. 547 Scanning tunneling microscopy is used to observe signatures of Majorana states at the ends of iron atom chains. [Also see Perspective by Lee] Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

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Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor

We have measured the resistance vs temperature of more than 20 superconducting nanowires with nominal widths ranging from 10 to 22 nm and lengths from 100 nm to 1 μm. With decreasing cross-sectional areas, the wires display increasingly broad resistive transitions. The data are in very good agreement with a model that includes both thermally activated phase slips close to Tc and quantum phase slips (QPS) at low temperatures, but disagree with an earlier model based on a critical value of Rn/Rq. Our measurements provide strong evidence for QPS in thin superconducting wires.

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Quantum Phase Slips in Superconducting Nanowires

The investigation of superconductivity in the presence of disorder began 60 years ago with the work of Alexander Shal'nikov at the Institute for Physical Problems in Moscow. The subject has played an ongoing role in condensed matter physics over the years. Interest has recently been heightened by the possibility that the disorder‐driven or magnetic‐field‐driven quenching of superconductivity in systems at the limit of zero temperature and two dimensions might be quantum phase transitions. That would link the physics of the superconductor‐insulator transition in thin films to other systems believed to exhibit quantum phase transitions—for example, helium‐4 in porous media, high temperature superconductors, Josephson‐junction arrays, two‐dimensional electron gases and various spin systems.

Superconductor‐Insulator Transitions in the Two‐Dimensional Limit

We have devised a scanned probe technique based on electrostatic force microscopy capable of probing the conductance of samples without requiring attached leads. We demonstrate that, using our technique, conductance can be probed on length scales as small as 0.4 Im. To demonstrate the utility of our technique, we use it to probe the conductance of DNA, a subject that has been the focus of intense debate with reported results ranging from metallic to insulating. In contrast to conducting single-wall carbon nanotubes, used as a control, individual strands of I-DNA, a widely studied sequence, are found to be insulating on the length scale probed.

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Scanned Conductance Microscopy of Carbon Nanotubes and λ-DNA

Experimental I-V curves of superconducting MoGe nanowires show hysteresis for the thicker wires and none for the thinner wires. A rather quantitative account of these data for representative wires is obtained by numerically solving the one-dimensional heat flow equation to find a self-consistent distribution of temperature and local resistivity along the wire, using the measured linear resistance R(T) as input. This suggests that the retrapping current in the hysteretic I-V curves is primarily determined by heating effects, and not by the dynamics of phase motion in a tilted washboard potential as often assumed. Heating effects and thermal fluctuations from the low-resistance state to a high-resistance, quasinormal regime appear to set independent upper bounds for the switching current.

Hysteretic I-V curves of superconducting nanowires

Oligopeptides bearing internal diacetylene units are shown to self-assemble in water into one-dimensional nanostructures and aligned macroscopic hydrogels. The diacetylene units can be photopolymerized into polydiacetylenes that run coincident to the nanostructure and noodle long axes, and the resulting nanostructures show evidence for ambipolar charge transport. This self-assembly, alignment and polymerization technique provides a rapid way to produce globally aligned collections of conjugated polymer chains.

Nina Markovic

Papers

Quantum Phase Slips in Superconducting Nanowires

Superconductor‐Insulator Transitions in the Two‐Dimensional Limit

Scanned Conductance Microscopy of Carbon Nanotubes and λ-DNA

Hysteretic I-V curves of superconducting nanowires

Synthesis and Alignment of Discrete Polydiacetylene-Peptide Nanostructures